This invention relates to lens grinding in general and more particularly to a blocking and mounting disc which provides heat shield protection which is particularly useful in grinding plastic lenses.
In former times, all lenses were made of glass. In the prescription eyewear field in particular, plastic lenses are now becoming quite popular because of their lightness and impact resistance. A common type of material used in such eyeglass lenses is a material known as CR-39, a registered trademark of P.P.G. Industries for an allyl diglycol carbonate monomer. The manner in which lens blanks made of this material are processed is described in detail in a booklet put out by P.P.G. Industries entitled "How to Surface Optical Plastic Lenses for Prescription Eyewear" [1975 P.P.G. Industries, Inc., Pittsburgh, Pa.]. In general, these steps include, after checking the prescription and selecting a proper blank, marking an axis on the lens to facilitate surface blocking, blocking the lens, i.e., placing it on a block to which it remains mounted during the subsequent processing, carrying out a step of generating in which special diamond wheels are used to remove large amounts of material and then carrying out a number of fining operations and polishing operations to obtain the final polished lens after which the lens is de-blocked. More detail regarding the various processes can be had from the aforementioned booklet. The present invention deals primarily with the blocking and de-blocking operations.
The conventional method of blocking has been to spray the lens with a tinted solvent based spray, place the lens on the block and supply blocking alloy through a hole in the opposite side of the block so that it fills an area between the lens and the block. The alloy bonds the lens blank to the block, permitting it to be further processed. This is essentially the same thing which is done in producing glass lenses. However, because the plastic is more sensitive to heat than is glass, a special lead alloy which melts at 117.degree. and is commonly called "low lead" is used as the alloy material. This is in contrast to the normal alloy having a melting point of approximately 158.degree. F. which is used with glass. At present, only approximately 20% of prescription lenses made are made of plastic. Thus, the optical laboratory must keep both types of alloy in stock. Furthermore, at the present time the "low lead" alloy costs approximately four times more than the normal alloy.
There have been some attempts in the prior art to avoid the need for using the special "low lead" alloy. These attempts have been through the use of a heat shield or blocking disc which is disposed between the lens and the block. One such disc comprises an opaque polyethylene material which is similar to the type used for pipe wrap having an adhesive on one side. The disc with the adhesive on one side is pressed onto the lens blank after which the other exposed side then must be sprayed with a lacquer similar to the tinted lacquer used in the process described above so that the alloy will properly adhere to the disc. (Without this layer, proper adhesion of the alloy would not take place). Aside from the fact that the additional step of spraying with lacquer is still required, the use of an opaque disc makes grinding and particularly inspection more difficult. With regard to problems with spraying, it should be noted that for spraying a special hooded spraying area is required and that, even with such, the danger of fire and to personnel from breathing fumes still exists. Other attempts have been made to overcome one of these problems, i.e., the opaqueness, through the use of clear discs for use as heat shields. However, these still have had only one side coated with adhesive and require the additional spraying step if the alloy is to properly adhere. Thus, although a saving can be attained in that normal rather than special alloy may be used for blocking, an increase in cost results because of the additional step required over that of the normal process, i.e. the step of applying the disc (not to mention the additional cost of the disc) with, no other steps eliminated. Thus, the need for an improved blocking disc which permits the use of the normal alloy with plastic lenses but which simplifies rather than adds to the complexity of the blocking process is evident.